This substance, although especially abundant in the liver, is rather widely distributed in the body, being a constituent of the white cells of the blood, the muscles, and various embryonic or growing and developing tissues. It is a white, tasteless substance resembling ordinary starch, except that on the addition of iodine it assumes a port-wine red instead of a blue colour. It appears that the liver can produce glycogen either from the saccharine or from the nitrogenous group of alimentary substances, but it is not increased when fats are given in excess. The livers of rabbits fed on carrots, and of dogs fed on lean meat, both contain abundance of glycogen. It is a store of easily oxidizable material, which, after being converted into sugar, can be absorbed by the blood at the liver, and being carried to the muscles is there burnt off, giving rise to the liberation of muscular force and to animal heat. It disappears both in muscles and in the liver after prolonged and violent exertion.
This gland, commonly but incorrectly spoken of as the "sweetbread", is deeply seated in the abdomen, lying above the stomach, between it and the spine, and occupies the space enclosed by the loop of the duodenum or first division of the small intestine. It is of pinkish-gray colour, and weighs about 1 1/2 lb. It is a highly important gland, its secretion exercising a powerful digestive action on starches, oils, and proteids.